1. Field of the Invention
This invention relates to video signal compensation, and in particular, it relates to circuitry and related methods for detecting and compensating for color separation in video signals transmitted over a cable with multiple transmission wires.
2. Description of the Related Art
In various systems, such as computer systems, where analog color video signals from a video source such as a computer is transmitted over relatively long transmission lines having multiple conductor lines such as a category 5 (Cat 5) cable, to a remote display device such as a monitor, the color components such as RG B components of the video signal often become slightly delayed relative to each other when they arrive at the display device. This is sometimes referred to as skew, and is typically caused by the slight length difference of the conductor wires in the cable that carry the different color components of the video signal. As a result, the color images displayed on the remote display device exhibit image degradation effects such as color separation.
Many technologies have been described for compensating for such skew. In one example, U.S. Pat. No. 7,113,012 describes a skew delay compensator which uses (as shown in its FIG. 3) a number of adjustable delay lines (DL1, DL2, DL3) connected to communication interfaces at the downstream end of the transmission conductors (3001, 3002, 3003). Connected to the delay lines are a detecting means (DD) for measuring propagation delay indicative parameters of the conductors, and a microprocessor (MP) for automatic adjustment of at least one adjustable delay line on the basis of said measured propagation delay indicative parameters so that the mutual delay between the conductors is minimized. A test oscillator 305 is provided at the upstream end of the conductors to generate a fully synchronized fixed frequency test signal. At the start of a calibration process, the test oscillator is activated to provide the test signal at the upstream end of the conductors, and the delay lines at the downstream end are adjusted so that the mutual delay in the test signals are minimized. The settings of the delay lines are stored in a memory, and the calibration process ends. Thereafter, the stored delay line settings are used to perform compensation for the RGB video signals.
In another example, U.S. Pat. Appl. Pub. 2008/0111643 (commonly owned as the present application) describes a signal delay compensation circuit (see its FIG. 1) which uses variable delay lines 114, a phase detector 118 and a controller 120 at the downstream end of the cable 106 to perform signal compensation. At the upstream end of the cable, an oscillator 112 generates an oscillation signal, and multiplexers 122 selects either the analog video signals or the oscillation signals to input into the transmission lines of the cable. During normal operation, the multiplexers select the analog video signals, and during a signal compensation process (which occurs at the beginning of the communication or when the user instructs to optimize the signal synchronization), the multiplexers select the oscillation signals to be inputted to the cable. The variable delay lines are adjusted during the signal compensation process and then used during normal operation to perform skew compensation.
In another example, U.S. Pat. No. 7,277,104 describes a device for reducing and determining the skew between color video signals transmitted over at least two different video cables. Skew detection circuits, each including a pulse separation detecting circuit and a pulse phase detecting circuit, are provided in the remote unit to measure the skew. To perform a skew compensation test, the video display is blanked, and detection signals, which include delay signals and phase signals, are applied to determine skew compensation. The video signal is reconnected afterwards. Another example, U.S. Pat. Appl. Pub. No. 2005/0132087, is similar in that the normal video signal transmission is interrupted in order to perform skew detection and compensation.